Papermaking machines for removing moisture from the source material of paper are generally equipped with a wire part, a press part and a dryer part. These parts are arranged in the order of wire part, press part and dryer part in the wet paper web transfer direction.
In one type of papermaking machine, the wet paper web is passed from one part to another in an open-draw. In the press part of this open-draw papermaking machine, there are a number of places in which the wet paper web is not supported by any roll or by papermaking equipment such as a felt or a belt; in other words, places in which the wet paper web is traveling on its own. In these places, problems such as “web breaks” tend to occur. The risk of these problems occurring increases as the papermaking machine is operated at higher speeds. Therefore, there are limitations to operating an open-draw papermaking machine at high speeds.
In recent years, most papermaking machines have been of the type in which the wet paper web is passed in a closed-draw. In the press part of this closed-draw papermaking machine, the wet paper web is transferred while being placed on a papermaking felt or wet paper web transfer belt. Therefore, there are no places in which the wet paper web travels on its own as in the open-draw papermaking machine. As a result, it has become possible to operate papermaking machines at even higher speeds and to stabilize operations.
Incidentally, in the press part of such a closed-draw papermaking machine, the so-called “paper robbing” phenomenon may occur, in which the wet paper web gets stuck at a belt or a felt when it is passed between the belts or felts and is not passed to the next belt or felt to which it ought to be passed. In conventional machines, when the “paper robbing” phenomenon occurs, it is necessary to temporarily stop the papermaking operation and to change the setting of the device so that the wet paper web is properly passed.
A number of studies have been made for improving the wet paper web transfer properties in the press part.
JP 06-057678 teaches a wet paper web transfer belt, in which a wet paper web contacting surface formed on the upper surface of a base (wet paper web side) is formed by an impermeable polymer coating layer and a lower surface of the base (roll side) is formed by a fibrous web. Particles with a higher hardness than the polymer coating are mixed in the impermeable polymer coating layer and the particles are made to protrude from the surface by such means as polishing the wet paper web contacting surface.
Moreover, the wet paper web contacting surface is a rough surface configured to be in the range of Rz=0 microns to 20 microns inside the press part and to recover to within the range of Rz=2 microns to 80 microns after exiting the press part.
The wet paper web transfer belt according to JP 06-057678 realizes to a high degree the adhesive and release properties of the wet paper web with the wet paper web contacting surface required to wet paper web transfer belts. Nevertheless, since different types of paper are made in the papermaking step, the basis weight of the paper naturally also differs. Therefore, the amount of moisture removed from the wet paper web during the pressing and the moisture content and amount of moisture of the wet paper web after pressing also differ. The moisture of the wet paper web after pressing has a big influence on the adhesive and release properties of the wet paper web in relation to the wet paper web contacting surface of the wet paper web transfer belt. From this point of view, the wet paper web transfer belt according to JP 06-057678 is not adequate for realizing the adhesive and release properties of the wet paper web for different types of paper (in particular paper of different basis weight).
Furthermore, US 2007/0074836 discloses a wet paper web transfer belt, characterized in that one of the alternative characteristics of wet paper web transfer belts such as surface roughness, bending strength, compressibility, recovery capacity can be continuously changed in the width direction of the wet paper web transfer belt in order to correspond to the papermaking machine specific profile.